The present invention relates to a radiation-sensitive positive photoresist composition, and more in detail, a positive photoresist composition containing a novolak resin obtained by condensing a mixture of m-cresol, p-cresol and xylenol with formaldehyde and a specified 1,2-naphthoquinone diazide photosensitive-material.
The progress of the degree of integration of the integrated circuit has been accelerated yearly, and this is an age of so-called VLSI having the integrated circuit of the integration rate of higher than 100,000, wherein the design of 1.5 .mu.m rule and further that of 1.0 .mu.m rule is required.
Accompanying with the above-mentioned progress, the requirement for the photolithography technique has come to be strict year after year. In the photolithography technique, the hitherto used photoresist is the negative photoresist obtained by adding a photo-crosslinking agent, namely, a bisazide compound, to a cyclized polyisoprene rubber. However, the resolution of this photoresist is limited by the swelling of the photoresist when it is subjected to development, and it is difficult to obtain a resolution of higher than 3 .mu.m.
The photoresist which can respond to the above-mentioned requirement is the positive photoresist. The positive photoresist composition contains an alkali-soluble phenols-formaldehyde-novolak resin together with a photosensitive substance, generally a substituted naphthoquinone diazide.
When naphthoquinone diazide is irradiated, it absorbs ultra-violet rays and is converted into a ketene via a carbene as follows. ##STR1## and the ketene reacts with water in the reaction system to form an indenecarboxylic acid. The positive photoresist utilizes the phenomenon that the thus formed indenecarboxylic acid dissolves in the aqueous alkaline solution of the developer.
As mentioned above, since the positive photoresist utilizes an aqueous alkali solution as the developer, the photoresist is not swollen in developing different from the case of negative photoresist and accordingly, it is possible to improve the resolution.
Namely, the positive photoresist has developed as the photoresist showing the non-swelling and the high resolution as compared to the negative photoresist, and has been used to the VLSI of 2 .mu.m rule.
However, along with the shrinking of the design of the VLSI to 1.5 m rule and further to 1.0 m rule, it has been elucidated that the form of the cross-section of the pattern of the conventional positive photoresist becomes narrower in the position near the upper surface and on the other hand, becomes wider in the position near the bottom, in other words, the photoresist has a pattern profile of a trapezoid.
Since the thickness of the membrane of the widened bottom part of the photoresist is small, such a thinner part of the membrane of the photoresist is also subjected to etching, and it is elucidated that the pattern has not enough to retain the dimension-reproducibility after etching. Accordingly, the offer of the photoresist which has the rectangular profile of the pattern even in the design of 1.5 to 1.0 .mu.m rule has been keenly demanded.
However, the actual situation has been that a photoresist which fulfills the above-mentioned specificities and does not impair the other specificities such as the sensitivity, the low unexposed film-thickness loss after developing and the heat-resistance has not yet developed.
As a result of the present inventors' studies, it has been found that a positive photoresist composition showing an extremely excellent pattern profile and a high sensitivity without losing the various specificities of the conventional positive photoresist is available by combining a specified photosensitive material of 1,2-naphthoquinone diazides with a specified novolak resin, and the present invention has been attained based on the findings.